docs: def serialize_rfc(attr, **kwargs):
          """Serialize Datetime object into RFC-1123 formatted string.
  
          :param Datetime attr: Object to be serialized.
          :rtype: str
          :raises: TypeError if format invalid.
          """
          try:
              if not attr.tzinfo:
                  _LOGGER.warning(
                      "Datetime with no tzinfo will be considered UTC.")
              utc = attr.utctimetuple()
          except AttributeError:
              raise TypeError("RFC1123 object must be valid Datetime object.")
  
          return "{}, {:02} {} {:04} {:02}:{:02}:{:02} GMT".format(
              Serializer.days[utc.tm_wday], utc.tm_mday,
              Serializer.months[utc.tm_mon], utc.tm_year,
              utc.tm_hour, utc.tm_min, utc.tm_sec)